Effects of Oxygen Additions to Argon Shielding Gas on GTA Weld Shape

In order to investigate the effect of oxygen additions on the weld shape in gas tungsten arc welding, bead-on-plate specimens were made of SUS304 stainless steel using O2-Ar mixed shielding gas with oxygen additions from 1 000 to 10 000 ppm. The weld bead cross-sections and the weld surface oxide layer were observed by optical microscopy after welding. The oxygen content in the weld metal was measured using an Oxygen/Nitrogen Analyzer. The weld depth/width ratio increases substantially as a result from the additions of oxygen to the argon shielding gas in the range of 3 000 to 5 000 ppm both for the 10 and 20 L/min shielding gas flow rates. When the oxygen addition contents are below 2 000 ppm or over 6 000 ppm, the weld D/W ratio decreases to approximately 0.2. The oxygen in the weld pool plays an important role as an active element affecting the Marangoni convection mode. The inward Marangoni convection occurs on the liquid pool surface when the oxygen in the weld is over 100 ppm, and hence the D/W ratio increases suddenly. The thicker oxide layer on the weld pool surface is not only a barrier for the oxygen to transfer and become a solute in the weld pool, but also prevents the weld pool from moving freely, and hence changes the weld pool shape.